Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define the magnetic tunnel junctions (MTJ) in each MRAM device, precise patterning steps including photolithography and reactive ion etching (RIE), ion beam etching (IBE) or their combination are usually involved. During RIE, high energy ions remove materials vertically in those areas not masked by photoresist, separating one MTJ cell from another. However, the high energy ions can also react with the non-removed materials, oxygen, moisture and other chemicals laterally, causing sidewall damage and lowering device performance. To solve this issue, pure physical etching techniques such as pure Ar RIE or ion beam etching (IBE) have been applied to etch the MTJ stack.
However, due to the non-volatile nature, pure physically etched conductive materials in the MTJ and bottom electrode can form a continuous path across the tunnel barrier, resulting in shorted devices. One solution to this is to form dielectric surrounded vias smaller than the MTJ connecting the MTJ and bottom electrode. This allows for a great over etch of the MTJ so that the metal re-deposition from the MTJ itself can be limited below the tunnel barrier; meanwhile, re-deposition from the bottom electrode is completely avoided. However, the via height, which represents the spacing between the MTJ and bottom electrode, is usually <50 nm, limited by the poor etch selectivity between the photoresist and via material. A new approach to further increase the via height is required if a greater MTJ over etch is needed to further reduce the metal re-deposition.
Several references teach over etching to form MTJ's, including U.S. Patent Applications 2018/0040668 (Park et al) and 2017/0125668 (Paranipe et al). Other references teach thin vias on wider metal layers, such as U.S. Pat. No. 8,324,698 (Zhong et al). All of these references are different from the present disclosure.